We just released version 10.2.6 of 'ArcGIS Runtime for .NET SDK', which now supports 3D and KML files on both Windows Store 8.1, Windows Phone 8.1 and WPF. This means you can now quickly build 3D applications that supports all these platforms. We also released the package on NuGet for your convenience.

As a demonstration how quick it is to create a Windows Phone and Windows Store 3D app, here's a little video creating a universal 3D app from scratch in a couple of minutes: